Ultrafast Photoinduced Charge Separation in Wide‐Band‐Capturing Self‐Assembled Supramolecular Bis(donor styryl)BODIPY–Fullerene Conjugates

A new series of self‐assembled supramolecular donor–acceptor conjugates capable of wide‐band capture, and exhibiting photoinduced charge separation have been designed, synthesized and characterized using various techniques as artificial photosynthetic mimics. The donor host systems comprise of a 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) containing a crown ether entity at the meso‐position and two styryl entities on the pyrrole rings. The styryl end groups also carried additional donor (triphenylamine or phenothiazine) entities. The acceptor host system was a fulleropyrrolidine comprised of an ethylammonium cation. Owing to the presence of extended conjugation and multiple chromophore entities, the BODIPY host revealed absorbance and emission well into the near‐IR region covering the 300–850 nm spectral range. The donor–acceptor conjugates formed by crown ether–alkyl ammonium cation binding of the host–guest system was characterized by optical absorbance and emission, computational, and electrochemical techniques. Experimentally determined binding constants were in the range of 1–2×10⁵ M ^?1. An energy‐level diagram to visualize different photochemical events was established using redox, computational, absorbance, and emission data. Spectral evidence for the occurrence of photoinduced charge separation in these conjugates was established from femtosecond transient absorption studies. The measured rates indicated ultrafast charge separation and relatively slow charge recombination revealing their usefulness in light‐energy harvesting and optoelectronic device applications. The bis(donor styryl)BODIPY‐derived conjugates populated their triplet excited states during charge recombination.     

Light‐Harvesting Ytterbium(III)–Porphyrinate–BODIPY Conjugates: Synthesis,Excitation‐Energy Transfer,and Two‐Photon‐Induced Near‐Infrared‐Emission Studies

Based on a donor–acceptor framework, several conjugates have been designed and prepared in which an electron‐donor moiety, ytterbium(III) porphyrinate (YbPor), was linked through an ethynyl bridge to an electron‐acceptor moiety, boron dipyrromethene (BODIPY). Photoluminescence studies demonstrated efficient energy transfer from the BODIPY moiety to the YbPor counterpart. When conjugated with the YbPor moiety, the BODIPY moiety served as an antenna to harvest the lower‐energy visible light, subsequently transferring its energy to the YbPor counterpart, and, consequently, sensitizing the Yb^III emission in the near‐infrared (NIR) region with a quantum efficiency of up to 0.73 % and a lifetime of around 40 μs. Moreover, these conjugates exhibited large two‐photon‐absorption cross‐sections that ranged from 1048–2226 GM and strong two‐photon‐induced NIR emission.     

From Dark to Light to Fluorescence Resonance Energy Transfer (FRET): Polarity‐Sensitive Aggregation‐Induced Emission (AIE)‐Active Tetraphenylethene‐Fused BODIPY Dyes with a Very Large Pseudo‐Stokes Shift

The work presented herein is devoted to the fabrication of large Stokes shift dyes in both organic and aqueous media by combining dark resonance energy transfer (DRET) and fluorescence resonance energy transfer (FRET) in one donor–acceptor system. In this respect, a series of donor–acceptor architectures of 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) dyes substituted by one, two, or three tetraphenylethene (TPE) luminogens were designed and synthesised. The photophysical properties of these three chromophore systems were studied to provide insight into the nature of donor–acceptor interactions in both THF and aqueous media. Because the generation of emissive TPE donor(s) is strongly polarity dependent, due to its aggregation‐induced emission (AIE) feature, one might expect the formation of appreciable fluorescence emission intensity with a very large pseudo‐Stokes shift in aqueous media when considering FRET process. Interestingly, similar results were also recorded in THF for the chromophore systems, although the TPE fragment(s) of the dyes are non‐emissive. The explanation for this photophysical behaviour lies in the DRET. This is the first report on combining two energy‐transfer processes, namely, FRET and DRET, in one polarity‐sensitive donor–acceptor pair system. The accuracy of the dark‐emissive donor property of the TPE luminogen is also presented for the first time as a new feature for AIE phenomena.     

BODIPY‐Based Fluorescent Thermometer as a Lysosome‐Targetable Probe: How the Oligo(ethylene glycols) Compete Photoinduced Electron Transfer

A novel BODIPY‐based fluorescent thermometer, which shows a lysosome‐targeting property, was successfully prepared. Due to the electron‐donating ability of the oligo(ethylene glycols), the photoinduced electron‐transfer pathway from morpholine to BODIPY dye is blocked. The fluorescence of the thermometer quenched by intramolecular rotation at room temperature was progressively enhanced during heating due to the increased microviscosity around the fluorophore.     

Synthesis of covalently linked boron-dipyrromethene-chromophore conjugates using 3-bromo boron-dipyrromethene as a key precursor

3-Bromo boron dipyrromethene (3-bromo BODIPY) has been used as key synthon to prepare one ethynyl bridged and six ethynylphenyl bridged BODIPY-chromophore conjugates using mild Pd(0) coupling conditions. The chromophores possessing very distinct features, such as anthracene, BODIPY, terpyridine, porphyrin, Zn(II)porphyrin, 21,23-dithiaporphyrin and thiasapphyrin were connected at 3-position of boronboron-dipyrromethene dye by coupling of 3-bromo BODIPY with ethynyl or ethynylphenyl chromophore in toluene/triethylamine in the presence of catalytic amount of AsPh₃/Pd₂(dba)₃ at 40 °C followed by column chromatographic purification. The spectral studies indicated that the interaction is stronger in ethynyl bridged BODIPY-chromophore conjugate compared to ethynylphenyl bridged BODIPY-chromophore conjugates. The steady-state fluorescence indicated that in ethynyl bridged BODIPY-anthracene conjugate, the BODIPY unit act as energy acceptor and showed a possibility of energy transfer from donor anthracene unit to acceptor BODIPY unit on selective excitation of anthracene unit. However, in ethynylphenyl bridged BODIPY-porphyrin conjugates, the BODIPY unit act as energy donor and exhibited a possibility of singlet-singlet energy transfer from BODIPY unit to porphyrin unit.     

A photochromic acceptor as a reversible light-driven switch in fluorescence resonance energy transfer (FRET)

A method has been developed for the quantitative determination of fluorescence resonance energy transfer (FRET) based on the modulation of donor fluorescence upon the reversible photoconversion of a photochromic acceptor. A model system was devised, consisting of Lucifer Yellow cadaverine (LYC, donor) conjugated to the photochromic molecule, 6-nitroBIPS (1′,3′-dihydro-1′-(2-carboxyethyl)-3′,3′-dimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indoline]). Near-ultraviolet irradiation catalyzes the conversion of the colorless spiropyran (SP) to the colored merocyanine (MC) form of 6-nitroBIPS. Only the MC form absorbs at the emission wavelengths of the donor, thereby potentiating FRET, as demonstrated by quenching of the donor. Subsequent irradiation in the visible MC absorption band reverts 6-nitroBIPS to the SP form and FRET is inactivated. The acceptor exhibited high photostability under repeated cycles of alternating UV–Vis irradiation. In this model system, the intramolecular FRET efficiency was close to 100%. The observed maximal donor quenching of 34±3% was indicative of an equilibrium determined by the high quantum efficiency of forward conversion (SP→MC) induced by near-UV irradiation and a low but finite quantum efficiency of the back reaction resulting from excitation of the MC form directly as well as indirectly (by FRET via the donor). A quantitative formalism for the photokinetic scheme was developed. Photochromic FRET (pcFRET) permits repeated, quantitative, and non-destructive FRET determinations for arbitrary relative concentrations of donor and acceptor and thus offers great potential for monitoring dynamic molecular interactions in living cells over extended observation times by fluorescence microscopy.     

Convenient and efficient FRET platform featuring a rigid biphenyl spacer between rhodamine and BODIPY: transformation of 'turn-on' sensors into ratiometric ones with dual emission

We have connected a borondipyrromethene (BODIPY) donor to the 5′ position of a tetramethylrhodamine (TMR) acceptor to form a high efficiency (over 99 %) intramolecular fluorescence resonance energy transfer (FRET) cassette, BODIPY–rhodamine platform (BRP). While the good spectral overlap between the emission of BODIPY and the absorption of TMR was one favorable factor, another feature of this FRET system was the rigid and short biphenyl spacer that favored efficient through‐bond energy transfer. More importantly, in this system, the 2′‐carboxyl group of the rhodamine unit was preserved for the further modifications, which was as convenient as those carbonyl groups on the original rhodamines without connection to donors. For this reason, BRP is clearly differentiated from the previous ratiometric sensors based on donor rhodamine systems. To illustrate its value as a versatile platform, we introduced typical Hg²⁺ receptors into BRP, through convenient one‐pot reactions on the 2′‐carboxyl group, and successfully developed two ratiometric sensors, BRP‐1 and BRP‐2, with different spirocyclic receptors that recognized Hg²⁺ on different reaction mechanisms. Upon excitation at a single wavelength (488 nm), at which only BODIPY absorbed, both of the FRET sensors exhibited clear Hg²⁺‐induced changes in the intensity ratio of the two strong emission bands of BODIPY and rhodamine. It should be noted that these ratiometric Hg²⁺ sensors exhibited excellent sensitivity and selectivity Hg²⁺, as well as pH insensitivity, which was similar to the corresponding ‘turn‐on’ rhodamine sensors. While both ratiometric probes were applicable for Hg²⁺ imaging in living cells, BRP‐1 exhibited higher sensitivity and faster responses than BRP‐2. Our investigation indicated that on a versatile platform, such as BRP, a large number of highly efficient ratiometric sensors for transition‐metal ions could be conveniently developed.     

A New Robust and Highly Sensitive FRET Donor–Acceptor Pair: Synthesis,Characterization, and Application in a Thrombin Assay

The synthesis of a new, robust fluorescence‐resonance‐energy‐transfer (FRET) system is described. Its donor chromophore is derived from an N‐allyl‐substituted quinolinone attached to 4‐bromophenylalanine via Heck cross‐coupling. The resulting Fmoc‐protected derivative 11 was used as building block in solid‐phase peptide synthesis (SPPS). As FRET acceptor, a sulfonylated ruthenium(II)–bathophenanthroline complex with a peripheral COOH function was prepared for covalent attachment to target molecules. The UV/VIS absorption and emission spectra of peptides bearing only the donor (D) or acceptor (A) dye showed a good overlap of the emission band of the donor with the absorption band of the acceptor. The fluorescence spectra of a peptide bearing both dyes revealed an additional emission after excitation of the donor, which is due to indirect excitation of the acceptor via FRET. The long fluorescence lifetime of the Ru^II complex (0.53 μs) makes it well‐suited for time‐resolved measurements. As a first application of this new FRET system, the peptide 18 , with the recognition sequence for the protease thrombin, flanked by the two dyes, was synthesized and successfully cleaved by the enzyme. The change in the ratio of the fluorescence intensities could be determined.     

Panchromatic Light Capture and Efficient Excitation Transfer Leading to Near‐IR Emission of BODIPY Oligomers

All‐BODIPY‐based (BODIPY=boron‐dipyrromethene) donor–acceptor systems capable of wide‐band absorbance leading to efficient energy transfer in the near‐IR region are reported. A covalently linked 3‐pyrrolyl BODIPY–BODIPY dimer building block bearing an ethynyl group at the meso‐aryl position is synthesized and coupled with three different monomeric BODIPY/pyrrolyl BODIPY building blocks with a bromo/iodo group under Pd⁰ coupling conditions to obtain three covalently linked 3‐pyrrolyl‐BODIPY‐based donor–acceptor oligomers in 19–29 % yield. The oligomers are characterized in detail by 1D and 2D NMR spectroscopy, high‐resolution mass spectrometry, and optical spectroscopy. Due to the presence of different functionalized BODIPY derivatives in the oligomers, panchromatic light capture (300–725 nm) is witnessed. Fluorescence studies reveal singlet–singlet energy transfer from BODIPY monomer to BODIPY dimer leading to emission in the 700–800 nm range. Theoretical modeling according to the Förster mechanism predicts ultrafast energy transfer due to good spectral overlap of the donor and acceptor entities. Femtosecond transient absorption studies confirm this to be the case and thus show the relevance of the currently developed all‐BODIPY‐based energy‐funneling supramolecular sytems with near‐IR emission to solar‐energy harvesting applications.     

Förster Resonance Energy Transfer Investigations Using Quantum‐Dot Fluorophores

F?rster resonance energy transfer (FRET), which involves the nonradiative transfer of excitation energy from an excited donor fluorophore to a proximal ground-state acceptor fluorophore, is a well-characterized photophysical tool. It is very sensitive to nanometer-scale changes in donor-acceptor separation distance and their relative dipole orientations. It has found a wide range of applications in analytical chemistry, protein conformation studies, and biological assays. Luminescent semiconductor nanocrystals (quantum dots, QDs) are inorganic fluorophores with unique optical and spectroscopic properties that could enhance FRET as an analytical tool, due to broad excitation spectra and tunable narrow and symmetric photoemission. Recently, there have been several FRET investigations using luminescent QDs that focused on addressing basic fundamental questions, as well as developing targeted applications with potential use in biology, including sensor design and protein conformation studies. Herein, we provide a critical review of those developments. We discuss some of the basic aspects of FRET applied to QDs as both donors and acceptors, and highlight some of the advantages offered (and limitations encountered) by QDs as energy donors and acceptors compared to conventional dyes. We also review the recent developments made in using QD bioreceptor conjugates to design FRET-based assays.     

Diameter‐Sorted SWCNT–Porphyrin and SWCNT–Phthalocyanine Conjugates for Light‐Energy Harvesting

A non‐covalent double‐decker binding strategy is employed to construct functional supramolecular single‐wall carbon nanotubes (SWCNT)–tetrapyrrole hybrids capable of undergoing photoinduced electron transfer and performing direct conversion of light into electricity. To accomplish this, two semiconducting SWCNTs of different diameters (6,5 and 7,6) were modified via π–π stacking of pyrene functionalized with an alkyl ammonium cation (PyrNH₃⁺). Such modified nanotubes were subsequently assembled via dipole–cation binding of zinc porphyrin with one ( 1 ) or four benzo‐18‐crown‐6 cavities ( 2 ) or phthalocyanine with four benzo‐18‐crown‐6 cavities at the ring periphery ( 3 ), employed as visible‐light photosensitizers. Upon charactering the conjugates using TEM and optical techniques, electron transfer via photoexcited zinc porphyrin and phthalocyanine was investigated using time‐resolved emission and transient absorption techniques. Higher charge‐separation efficiency is established for SWCNT(7,6) with a narrow band gap than the thin SWCNT(6,5) with a wide band gap. Photoelectrochemical studies using FTO/SnO₂ electrodes modified with these donor–acceptor conjugates unanimously demonstrated the ability of these conjugates to convert light energy into electricity. The photocurrent generation followed the trend observed for charge separation, that is, incident‐photon‐to‐current efficiency (IPCE) of a maximum of 12 % is achieved for photocells with FTO/SnO₂/SWCNT(7,6)/PyrNH₃⁺: 1 .     

Selective Detection of Hg2+ Ions with Boron Dipyrromethene‐Based Fluorescent Probes Appended with a Bis(1,2,3‐triazole)amino Receptor

By using a copper‐promoted alkyne–azide cycloaddition reaction, two boron dipyrromethene (BODIPY) derivatives bearing a bis(1,2,3‐triazole)amino receptor at the meso position were prepared and characterized. For the analogue with two terminal triethylene glycol chains, the fluorescence emission at 509 nm responded selectively toward Hg²⁺ ions, which greatly increased the fluorescence quantum yield from 0.003 to 0.25 as a result of inhibition of the photoinduced electron transfer (PET) process. By introducing two additional rhodamine moieties at the termini, the resulting conjugate could also detect Hg²⁺ ions in a highly selective manner. Upon excitation at the BODIPY core, the fluorescence emission of rhodamine at 580 nm was observed and the intensity increased substantially upon addition of Hg²⁺ ions due to inhibition of the PET process followed by highly efficient fluorescence resonance energy transfer (FRET) from the BODIPY core to the rhodamine moieties. The Hg²⁺‐responsive fluorescence change of these two probes could be easily seen with the naked eye. The binding stoichiometry between the probes and Hg²⁺ ions in CH₃CN was determined to be 1:2 by Job′s plot analysis and ¹H NMR titration, and the binding constants were found to be (1.2±0.1)×10¹¹ m ^?2 and (1.3±0.3)×10¹⁰ m ^?2, respectively. The overall results suggest that these two BODIPY derivatives can serve as highly selective fluorescent probes for Hg²⁺ ions. The rhodamine derivative makes use of a combined PET‐FRET sensing mechanism which can greatly increase the sensitivity of detection.     

Preparation of an IMI dye (imidazole functional group) containing a 4-(N,N-dimethylaminosulfonyl)-2,1,3-benzoxadiazole fluorophore for labeling of phosphomonoesters

We are studying dye-imidazole conjugates ("IMI dyes") as reagents for labeling phosphomonoesters such as nucleotides. Previously we have employed a BODIPY dye in our IMI reagents, and analyzed the labeled products by capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) involving an argon ion laser. (The BODIPY fluorophore is a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene). Here we broaden the technology by preparing a DBD-IMI dye [DBD = 4-(N,N-dimethylaminosulfonyl)-2,1,3-benzoxadiazole], and using a helium-cadmium laser. While DBD-IMI (IMI3) is about 50x more stable photolytically than a BODIPY-IMI dye (IMI2, a conjugate of a BODIPY dye with histamine, was tested), the detection limit for IMI2 (5.10(-11) M; S/N = 5, CE-LIF with an argon ion laser) is tenfold better than that for IMI3 (5.10(-10) M, S/N = 5, helium-cadmium laser). IMI3 conjugates of the four major DNA nucleotides were prepared and detected by CE-LIF.     

Oligo(p-phenylene ethynylene)-BODIPY derivatives: synthesis, energy transfer, and quantum-chemical calculations

The synthesis and energy‐transfer properties of a series of oligo(p‐phenylene ethynylene)–BODIPY ( OPEB ) cassettes are reported. A series of oligo(p‐phenylene ethynylene)s ( OPE s) with different conjugated chain lengths as energy donor subunit in the energy‐transfer system were capped at both ends with BODIPY chromophores as energy‐acceptor subunits. The effect of the conjugated chain of OPE s on energy transfer in the OPEB cassettes was investigated by UV/Vis and fluorescence spectroscopy and modeling. With increasing number n of phenyl acetylene units (n=1–7), the absorption and emission maxima of OPEn are bathochromically shifted. In the OPEBn analogues, the absorption maximum assigned to the BODIPY moieties is independent of the length of the OPE spacer. However, the relative absorption intensity of the BODIPY band decreases when the number of phenyl acetylene units is increased. The emission spectra of OPEBn are dominated by a band peaking at 613 nm, corresponding to emission of the BODIPY moieties, regardless of whether excitation is at 420 or 550 nm. Furthermore, a very small band is observed with a maximum between 450 and 500 nm, and its intensity relative to that of the BODIPY emission increases with increasing n, that is, the excited state of OPE subunits is efficiently quenched in OPEBn by energy transfer to the BODIPY moieties. Energy transfer (ET) from OPEn to BODIPY in OPEBn is very efficient (all Φ_ET values are greater than 98 %) and only slightly decreases with increasing length of the OPE units. These results are supported by theoretical studies that show very high energy transfer efficiency (Φ_ET>75 %) from the OPE spacer to the BODIPY end‐groups for chains with up to 15–20 units.     

Broadband Visible‐Light‐Harvesting trans‐Bis(alkylphosphine) Platinum(II)‐Alkynyl Complexes with Singlet Energy Transfer between BODIPY and Naphthalene Diimide Ligands

A heteroleptic bis(tributylphosphine) platinum(II)‐alkynyl complex ( Pt‐1 ) showing broadband visible‐light absorption was prepared. Two different visible‐light‐absorbing ligands, that is, ethynylated boron‐dipyrromethene (BODIPY) and a functionalized naphthalene diimide (NDI) were used in the molecule. Two reference complexes, Pt‐2 and Pt‐3 , which contain only the NDI or BODIPY ligand, respectively, were also prepared. The coordinated BODIPY ligand shows absorption at 503 nm and fluorescence at 516 nm, whereas the coordinated NDI ligand absorbs at 594 nm; the spectral overlap between the two ligands ensures intramolecular resonance energy transfer in Pt‐1 , with BODIPY as the singlet energy donor and NDI as the energy acceptor. The complex shows strong absorption in the region 450 nm–640 nm, with molar absorption coefficient up to 88 000 M ^?1 cm^?1. Long‐lived triplet excited states lifetimes were observed for Pt‐1 – Pt‐3 (36.9 μs, 28.3 μs, and 818.6 μs, respectively). Singlet and triplet energy transfer processes were studied by the fluorescence/phosphorescence excitation spectra, steady‐state and time‐resolved UV/Vis absorption and luminescence spectra, as well as nanosecond time‐resolved transient difference absorption spectra. A triplet‐state equilibrium was observed for Pt‐1 . The complexes were used as triplet photosensitizers for triplet–triplet annihilation upconversion, with upconversion quantum yields up to 18.4 % being observed for Pt‐1 .     

Efficient Intramolecular Energy Transfer between Two Fluorophores in a Bis‐Branched [3]Rotaxane

A bis‐branched [3]rotaxane, with two [2]rotaxane arms separated by an oligo(para‐phenylenevinylene) (OPV) fluorophore, was designed and investigated. Each [2]rotaxane arm employed a difluoroboradiaza‐s‐indacene (BODIPY) dye‐functionalized dibenzo[24]crown‐8 macrocycle interlocked onto a dibenzylammonium in the rod part. The chemical structure of the [3]rotaxane was confirmed and characterized by ¹H and ¹³C NMR spectroscopy and high‐resolution ESI mass spectrometry. The photophysical properties of [3]rotaxane and its reference systems were investigated through UV/Vis absorption, fluorescence, and time‐resolved fluorescence spectroscopy. An efficient energy‐transfer process in [3]rotaxane occurred from the OPV donor to the BODIPY acceptor because of the large overlap between the absorption spectrum of the BODIPY moiety and the emission spectrum of the OPV fluorophore; this shows the important potential of this system for designing functional molecular systems.     

Synthesis,Structure, and Optical Studies of Donor–Acceptor‐Type Near‐Infrared (NIR) Aza–Boron‐Dipyrromethene (BODIPY) Dyes

Six donor–acceptor‐type near‐infrared (NIR) aza–boron‐dipyrromethene (BODIPY) dyes and their corresponding aza–dipyrrins were designed and synthesized. The donor moieties at the 1,7‐positions of the aza–BODIPY core were varied from naphthyl to N‐phenylcarbazole to N‐butylcarbazole. The 3,5‐positions were also substituted with phenyl or thienyl groups in the aza–BODIPYs. Photophysical, electrochemical, and computational studies were carried out. The absorption and emission spectra of aza–BODIPYs were significantly redshifted (≈100 nm) relative to the parent tetraphenylaza–BODIPY. Fluorescence studies suggested effective energy transfer (up to 93 %) from donor groups to the aza–BODIPY core in all of the compounds under study. Time‐dependent (TD)‐DFT studies indicated effective electronic interactions between energy donor groups and aza–dipyrrin unit in all the aza–BODIPYs studied. The HOMO–LUMO gap (ΔE) calculated from cyclic voltammetry data was found to be lower for six aza–BODIPYs relative to their corresponding aza–dipyrrins.     

A near IR di-styryl BODIPY-based ratiometric fluorescent chemosensor for Hg(II)

A novel BODIPY-based near-IR emitting probe as a selective and sensitive fluorophore for Hg(II) is synthesized. This versatile BODIPY fluorophore is functionalized for long wavelength emission at the 3 and 5 positions via a condensation reaction in which two dithiodioxomonoaza-based crown-containing phenyl units are conjugated to the BODIPY core as a chelating unit. This designed fluorophore, employing an ICT sensor can be used effectively to detect Hg(II) cations by way of a hypsochromic shift (∼90 nm) in both the absorption and emission spectra.     

Systematic Modulation of the Fluorescence Brightness in Boron‐Dipyrromethene (BODIPY)‐Tagged N‐Heterocyclic Carbene (NHC)–Gold–Thiolates

Five different highly fluorescent boron‐dipyrromethene (BODIPY)‐tagged N‐heterocyclic carbene NHC–gold halide complexes were synthesized. The substitution of the halogeno ligand by 4‐substituted aryl thiolates leads to a decrease in the brightness of the complexes. This decrease depends on the electronic nature of the thiols, being most pronounced with highly electron‐rich thiols (4‐R=NMe₂). The brightness of the gold thiolates also depends on the distance between the sulfur atom and the BODIPY moiety. The systematic variation of the electron density of [(NHC–bodipy)Au(SC₆H₄R)] (via different R groups) enables the systematic variation of the fluorescence brightness of an appended BODIPY fluorophore. Based on this and supported by DFT calculations, a photoinduced electron‐transfer quenching appears to be the dominant mechanism controlling the brightness of the appended BODIPY dye.